In the oil and gas industries, disconnect devices are typically used to separate a bottom hole assembly (BHA) from a drill string if, for example, the BHA becomes stuck. Once the drill string has been disconnected from the BHA, the operators can then attempt to recover the stuck BHA with a “fishing” tool. However, in situations where recovery of the BHA is impractical or impossible, the stuck BHA will be abandoned and drilling will recommence along a different route with a new BHA attached to the drill string.
Typical methods for disconnecting a drill string from a stuck BHA involve dropping a dart, ball or mud slug of high density fluid from the surface to interact with a shear pin or other locking device and actuate the separation. For example, WO-A-03/029605 (Weatherford/Lamb, Inc.) describes a disconnect device having two portions connected by a lock nut. The two portions separate when a predetermined fluid force is applied to a piston in the disconnect device causing a tensile sleeve to fail. In one particular embodiment, the tensile sleeve's failure permits an annular piston to dislodge a wedge sleeve from the lock nut, thereby permitting separation. Such arrangements require the circulation of drilling mud to transport the interacting article (dart, ball or mud slug). However, this is often impossible when the BHA becomes stuck. Another disconnect device that relies on the circulation of fluid is described in GB-B-2351101. The GB-B-2351101 device comprises a radially expandable locking ring that is configured to expand and thereby disconnect the device.
Alternatively, drill strings can be separated without using specialist tools by performing a precise series of “back off” movements and rotations such as turning the drill string leftward and overpulling to affect a release. This technique is often complicated and difficult and is consequently unreliable.
A third option is to separate the drill string above the point at which it is stuck by explosive means. US-A-2004/0200343 (Titan Specialties, Ltd.) describes a pipe severing tool that is positioned into a well bore before exploding to actuate separation. The tool comprises explosive pellets and electrically initiated exploding wire detonators (EBW) that are positioned at opposite ends of a tubular housing for simultaneous detonation by a capacitive firing device.
This technique is often used as a last resort and usually requires the skills of a specialist team which may take several days to arrive at the rig and sever the drill string. Due to the high operating costs of drill rigs, this significant time period of non-operation can lead to substantial financial losses which are highly undesirable. Additionally, the damaged end of the drill string must be replaced before a new BHA can be connected and drilling can recommence. Furthermore, most explosive disconnection techniques are dependent upon gravity for locating the explosives close to the point at which the tool is stuck. It follows that explosive disconnection is generally not an option for the disconnection of a BHA in a horizontal section of the well bore.
There is therefore a need to provide a disconnect device that allows for a controlled disconnect from the BHA with no physical input from the surface other than mechanical signals. The present invention satisfies this need and allows for the drill string to be retracted undamaged so that drilling can recommence as quickly and as easy as possible following the disconnection. It is a further object of the present invention to provide a secure disconnect device that will only actuate when the tool is stuck and the operator wishes to do so.
It is a further object to provide a tool that is actuated by controlled movements of the tool without other signalling from the surface so that tools such as circulating subs can be reliably activated when required.